Rare-earth permanent magnets are commonly used in electric and electronic equipment on account of their excellent magnetic properties and economy. Lately there is an increasing demand to enhance their performance.
To enhance the magnetic properties of R—Fe—B based rare earth permanent magnets, the proportion of the R2Fe14B1 phase present in the alloy as a primary phase component must be increased. This means to reduce the Nd-rich phase as a nonmagnetic phase. This, in turn, requires to reduce the oxygen, carbon and nitrogen concentrations of the alloy so as to minimize oxidation, carbonization and nitriding of the Nd-rich phase.
However, reducing the oxygen concentration in the alloy affords a likelihood of abnormal grain growth during the sintering process, resulting in a magnet having a high remanence Br, but a low coercivity iHc, insufficient energy product (BH)max, and poor squareness.
The inventor disclosed in JP-A 2002-75717 (U.S. Pat. No. 6,506,265, EP 1164599A) that even when the oxygen concentration during the manufacturing process is reduced for thereby lowering the oxygen concentration in the alloy for the purpose of improving magnetic properties, uniform precipitation of ZrB, NbB or HfB compound in a fine form within the magnet is successful in significantly broadening the optimum sintering temperature range, thus enabling the manufacture of Nd—Fe—B base rare earth permanent magnet material with minimal abnormal grain growth and higher performance.
For further reducing the cost of magnet alloys, the inventor attempted to manufacture magnet alloys using inexpensive raw materials having high carbon concentrations and obtained alloys with significantly reduced iHc and poor squareness, i.e., properties not viable as commercial products.
It is presumed that such substantial losses of magnetic properties occur because in the existing ultra-high performance magnets having the R-rich phase reduced to the necessary minimum level, even a slight increase in carbon concentration can cause a substantial part of the R-rich phase which has not been oxidized to become a carbide. Then the quantity of the R-rich phase necessary for liquid phase sintering is extremely reduced.
The neodymium-base sintered magnets commercially manufactured so far are known to start reducing the coercivity when the carbon concentration exceeds approximately 0.05% and become commercially unacceptable in excess of approximately 0.1%.